1. Field of the Invention
The present invention relates to a dog clutch mechanism for selectively connecting and disconnecting input and output shafts which are coaxial with each other, and more particularly to a dog clutch mechanism for selectively connecting and disconnecting smoothly with a small force while drive power is being transmitted from the input shaft to the output shaft.
2. Description of the Prior Art
Various all-wheel-drive vehicles have been developed in recent years for higher stability and maneuverability during travel of the vehicles. One type of such all-wheel-drive vehicles is known as a part-time four-wheel-drive vehicle which can switch, as desired, between a two-wheel mode in which front or rear wheels serve as drive wheels under ordinary conditions, e.g., when running on the road, and a four-wheel mode in which drive torque is also transmitted to driven wheels under other conditions, e.g., when running off the road. All-wheel-drive vehicles of this type have a clutch on a rotatable drive shaft in order to selectively transmit drive torque from a power source via the rotatable drive shaft to the driven wheels.
Japanese Laid Open Patent Publication No. 58-56923, for example, discloses a part-time four-wheel-drive vehicle having a clutch mechanism comprising an electromagnetic clutch. Under normal running conditions, the vehicle operates as a front-wheel-drive vehicle in which only the front wheels are coupled to a drive shaft connected to a power source such as an engine. The drive shaft is connected as an input shaft to one end of the electromagnetic clutch. When the electromagnetic clutch is actuated as required, drive power can be transmitted from the drive shaft via the electromagnetic clutch to the rear wheels which are operatively coupled to the other end of the clutch, so that the vehicle can operate as a four-wheel-drive vehicle.
Japanese Laid-Open Utility Model Publication No. 55-1956 shows an all-wheel-drive vehicle equipped with a clutch mechanism comprising a dog clutch. The dog clutch is selectively engaged and disengaged to switch the vehicle between two-wheel-drive and four-wheel-drive modes.
The dog clutch disclosed in the latter publication differs from a friction clutch, such as the electromagnetic clutch disclosed in the former publication, in that since the dog clutch has meshing teeth for torque transmission, its structure is simple, no slippage whatsoever occurs, and reliable torque transmission is possible. The dog clutch however has problems in that it generates large shocks when it is engaged and disengaged, and requires a large operating force for disconnecting the shafts while torque is being transmitted between these shafts. Conventional dog clutches used to transmit and cut off drive power between coaxial input and output shafts have intermeshing dog teeth with their meshing surfaces lying parallel to the input and output shafts. Therefore, a very large operating force which is strong enough to overcome the meshing force of the dog teeth is necessary for disengaging the meshing dog teeth. At times, disengagement of the clutch may produce noise when dog teeth ride over confronting dog teeth, as well as produce a shock force.